1. Field of the Technology
The technology presented herein relates to a photoelectric conversion device and a manufacturing method thereof. More specifically, the technology relates to a photoelectric conversion device with improved photoelectric conversion efficiency and a manufacturing method thereof.
2. Description of the Related Art
Due to a problem with a decrease in quantity of remaining resources, there has been a concern about future supply deficiency in fossil fuels, such as oil, that are utilized as power energy sources. There has also been an issue with carbon dioxide emission that causes global warming. Under such circumstances, solar cells are attracting attention as alternative energy to fossil fuels.
In a typical solar cell, a p-n junction of a semiconductor is applied to a photoelectric conversion layer for converting light energy into electrical energy, and silicon is in the widest use as a raw material for the photoelectric conversion layer. Among solar cells using silicon, the most widely available ones have been solar cells using bulk crystalline silicon, such as monocrystalline silicon and polycrystalline silicon. Recently, prices for solar cell modules are decreasing due solar energy generation system is rapidly becoming widespread.
However, since a bulk-crystal type solar cell is formed using a silicon substrate having a thickness of several hundreds of micrometers, raw material cost forms a large proportion of a price for a solar cell, and it is thus becoming more difficult to significantly reduce the cost of producing the cell. Further, in production of a monocrystalline silicon substrate and a polycrystalline silicon substrate, which are used for the above-mentioned bulk-crystal type solar cell, the silicon needs to be heated at a high temperature of 1500° C. or higher for melting. This has caused a problem with a large volume of carbon dioxide emissions at the time of producing those silicon substrates.
Meanwhile, a technique for thin-film silicon solar cells is under development, as a next generation technique in which a silicon thin film is deposited on a substrate to significantly reduce an amount of silicon used, thus enabling significant reduction in cost as well as volume of carbon dioxide emissions.
A photoelectric conversion layer in the thin-film silicon solar cell is normally formed of a thin film of a semiconductor such as hydrogenated amorphous silicon or hydrogenated microcrystalline silicon. The solar cell formed of hydrogenated microcrystalline silicon is superior to the solar cell formed of hydrogenated amorphous silicon in that no light degradation occurs. Hence the solar cell formed of hydrogenated microcrystalline silicon is currently attracting attention as a thin-film silicon solar cell whose efficiency can further be enhanced.
The term “amorphous” is used in this specification as a synonymous with a term “amorphous” normally used in the field. Further, the term “microscopic crystal” used in this specification means not only a state formed substantially of a crystalline phase alone, but also a state where an amorphous phase and a crystalline phase are mixed. For example, in Raman scattering spectrum, if a sharp peak in the vicinity of 520 cm−1, attributed to silicon-silicon bonding in crystalline silicon, is detected even in a trace amount, the silicon in use is regarded as “microscopic crystal silicon”. In the same meaning, the term “microscopic crystal silicon” is used in the specification.
Typical thin-film silicon solar cells are classified into two types: a superstrate type and a substrate type. The superstrate type is a type of the thin-film silicon solar cell in a light transmitting conductive film, a photoelectric conversion layer and an electrode are laminated in this order on a light transmitting substrate and in which light is incident on the substrate side. The substrate type is type of the thin-film silicon solar cell in an electrode, a photoelectric conversion layer, a light transmitting conductive film and a grid electrode are laminated in this order on a substrate and in which light is incident on the grid electrode side. In many cases, the photoelectric conversion layer of both the superstrate and substrate types have a pin junction structure comprising a p-type semiconductor layer (p-type conductivity layer), an intrinsic semiconductor layer (i-type conductivity layer) and an n-type semiconductor layer (n-type conductivity layer).
However, since the thin-film silicon solar cell has low photoelectric conversion efficiency per unit area as compared to the bulk-crystal type solar cell, the market for the thin-film silicon solar cell has in reality not expanded like the market for the bulk-crystal type solar cell.
Therefore, enhancing efficiency is an important subject for the thin-film silicon solar cell to become fully widespread. One of means for enhancing efficiency may be widening a band gap of a semiconductor layer (hereinafter referred to as window layer) on a light incident plane side of a solar cell. In the above thin-film silicon solar cell, with the band gap of a window layer widened, a short-circuit current density increases due to a reduction in light absorption loss in the window layer, and an open-circuit voltage increases due to an increase in diffusion potential, thereby making it possible to enhance photoelectric conversion efficiency.
Examples of conventional techniques related to widening of a band gap of a window layer as thus described, include a technique for a thin-film photoelectric conversion device, described in JP-A 2002-016271. According to this thin-film photoelectric conversion device, light absorptions of a p-type semiconductor layer and an n-type semiconductor layer are reduced, and a band gap of an interface is widened, to reduce interface recombination, and thereby high photoelectric conversion efficiency can be obtained.
Examples of the conventional techniques further include, as a similar technique to the technique of JP-A 2002-016271, a technique for a polycrystalline silicon thin film described in Japanese Patent No. 3377814. This polycrystalline silicon thin film is obtained by forming a silicon film on a substrate where the nucleus of microcrystalline silicon has been formed. The nucleus of microcrystalline silicon is made out of a thin-film of a-SiC:H or a-SiN:H formed on the substrate.
The technique of JP-A 2002-016271 is to widen a band gap of a p-type semiconductor layer for enhancing efficiency. Meanwhile, a similar effect to this technique can be expected by widening a band gap of an n-type semiconductor layer, which is described in JP-A 2002-009313. In a pin-junction type thin-film solar cell formed by laminating a plurality of semiconductor layers, the solar cell is characterized by having a pin-structure which is mainly composed of amorphous silicon and whose n-type semiconductor layer has a band gap made wider than that of an i-type semiconductor layer, to contain a trace amount of n-type impurity in the i-type semiconductor layer.
In the technique of JP-A 2002-016271, when carbon is used as an added element for widening of a band gap, it is required to contain carbon in a large amount, not less than 10 atomic %, in a silicon film. However, this involves a simultaneous increase in uncombined silicon in the film. In other words, addition of an impurity at a high concentration of not lower than 10 atomic % leads to an increase in density of uncombined silicon formed in the film as compared to the case of a low-concentration impurity. Since the conductivity decreases with increase in defect density of the semiconductor layer, a fill factor of a solar cell might decrease. Further, it is known that, when a conductivity type determination element to be doped in a p-type semiconductor layer or an n-type semiconductor layer is activated in the film as a dopant, the activation efficiency of the element decreases with increase in concentration of carbon as the impurity element in the film. Hence, higher concentration of the impurity added, a carrier concentration decreases. Moreover, the lower the concentration of the impurity element in the film, the more crystallization tends to be enhanced, and under the above-mentioned high-concentration condition, a proportion of an amorphous phase in the p-type semiconductor layer or n-type semiconductor layer may increase.
In the technique of Japanese Patent No. 3377814, a thin film of a-SiC:H or a-SiN:H, containing microcrystalline silicon, is first formed on a substrate, the thin film is then removed by etching while remaining microcrystalline silicon alone, the nucleus of microcrystalline silicon is made to be exposed to the surface of the substrate, on which a silicon thin film is deposited again, to form a polycrystalline silicon thin film having a large particle size. Since the initially formed film of a-SiC:H or a-SiN:H is removed, leaving the nucleus of microcrystalline silicon, there may be some effect by widening of a band gap with carbon atoms and nitrogen atoms. However, the above effect is not exerted throughout the ultimately obtained polycrystalline silicon thin film. Namely, the foregoing effects of reduction in light absorption loss and improvement of an open-circuit voltage, produced by widening of a gap band, cannot be expected.
In the technique of JP-A 2002-009313, when an i-type semiconductor layer does not contain, especially, an n-type impurity, and thus an internal electric field is thus not weakened at an i/n interface, a material for widening of a band gap is used or an n-layer. However, widening of the band gap of the n-type semiconductor layer has not been able to bring about production of an equivalent effect of improving photoelectric conversion efficiency to the effect achieved by widening the band gap of the p-type semiconductor layer.